Resistance material and resistor made therefrom



Oct. 27, 1964 B. v. JANAKlRAMA-RAO ETAL 0 RESISTANCE MATERIAL ANDRESISTOR MADE THEREFROM Filed Jan. 12, 1961 /6'- GLASS CO/VMM/M/G ION/CS/LVER. GOLD OR COPPER MA T E R/AL e w m m R IZ- CERAMIC BASE INVENTORSBHOGARAJU K JANA/(IRAMA IMO REM/N6 T O/V MURPHY ATTORNEY United StatesPatent 3,154,503 RESESTANCE MATERIAL AND RESETGR MADE THEREFRQMBhogaraiu V. .lanalrirama-Rao, Philadelphia, and Reinington Murphy,Roslyn, lla., assignors to lnternaticnal Resistance Company,Philadelphia, Pa.

Filed Jan. 12, 1961, Ser. No. 82,164 6 Claims. (Cl. 252-514.)

The present invention relates to a resistance material and a resistormade therefrom. More particularly, the present invention relates to anenamel type resistance material and a resistor made therefrom.

One type of electrical resistor comprises a base or substrate of anelectrical insulating material, such as a ceramic, coated with a layerof a resistance material. The resistance material generally comprisesfine particles of a conductive material, such as carbon or a metal,suspended in a carrier material. It has been recently suggested to forman enamel type resistance material in which the carrier is a glass fritand the conductive material is fine particles of a metal or a mixture ofmetals. The conductive metals which have been found particularly goodfor use in the enamel type resistance material are the noble metals,such as gold, silver, platinum, palladium and rhodium and mixtures ofsuch noble metals. Resistors are formed from such glass frit, conductivemetal resistance enamels by coating a base or substrate with the enameland heating the unit to fire the glass frit, and thereby bond theresistance material to the base.

However, a number of problems have arisen with the enamel typeresistance material compositions so far suggested. It has been foundthat the glass frit when fired has poor wettability. The poorwettability of the glass frit provides poor adhesion between the enameland the base or substrate. Also, it provides poor adhesion between theglass and the conductive metal particles, and poor statisticaldistribution of the metal particles in the glass, both of whichadversely affect the electrical properties of the enamel resistancematerial. Another problem with such resistance enamels arises from thehigh cost of the noble metals used therein. In order to obtain theproper conductivity of the resistance enamels, the enamel must contain apercentage of the noble metal which is large when considering the costof the metal.

It is an object of the present invention to provide a novel resistancematerial.

It is another object to provide a novel enamel type resistance material.

It is still another object of the present invention to provide an enameltype resistance material including a glass frit having improvedwettability.

It is a further object of the present invention to provide an enameltype resistance material which uses smaller amounts of the conductivemetal to decrease the cost of the material.

It is still a further object of the present invention to provide animproved electrical resistor having an enamel type resistance coating.

Other objects will appear hereinafter.

The invention accordingly comprises a composition of matter possessingthe characteristics, properties, and the relation of constituents whichwill be exemplified in the composition hereinafter described, and thescope of the invention will be indicated in the claims.

The drawing is a cross-sectional view, on a highly exaggerated scale, ofa resistor produced in accordance with the present invention.

In general, the enamel type resistance material of the present inventioncomprises a vitreous frit containing either silver, gold, copper ormixtures thereof in the ionic 3,.l5th5h3 Patented Get. 27, 1964 statemixed with a finely divided conductor of palladium, platinum, rhodium ormixtures thereof. The proportions of the glass frit and the finelydivided conductor in the enamel resistance material of the presentinvention may be by weight 99% to 50% glass frit, and 1% to 50%conductor. The glass frit may be a borosilicate frit, lead borosilicatefrit, cadmium, barium, calcium or oher borosilicate frit which containsthe silver, gold, copper or mixture thereof in the ionic state. Theglass frit may contain from 1% to 25% of the silver, gold, copper ormixture thereof calculated by weight of the oxide of the additionalmetal.

To form a resistor, the enamel resistance material of the presentinvention may be applied to a substrate or base of an electricalinsulating material, such as a ceramic, and fired to fuse the glass fritand bond the frit and the conductor to the substrate or base. For easeof application of the enamel resistance material to the base orsubstrate, the enamel composition may be mixed with a vehicle, such asbutyl carbitol acetate or toluol, to form a liquid or paste. The liquidor paste can then be applied to the base or substrate by dip coating,spraying, screen printing or roller coating.

As shown in the drawing, a resultant resistor of the present inventionis generally designated as 10. Resistor It) comprises the ceramic baseor substrate 12 having a layer 14 of the resistance material of thepresent invention coated and fired thereon. The resistance materiallayer 14 comprises the glass 16 containing the silver, gold, copper ormixture thereof in the ionic state, and the finely divided metalparticles 18 dispersed throughout the glass 16.

As previously stated, the glass frit of the enamel resistance materialof the present invention may be a borosilicate frit, lead borosilicatefrit, cadmium, barium, calcium, or other borosilicate frit whichcontains silver, gold or copper in the ionic state. Examples of somepreferred glass frits which can be used with the proportions being byweight are as follows:

Percent PbO to 80 B203 1 to S10 10 to 40 Ag O 1 to 24 BaO 40 to CaO 1 to15 B 0 10 to 35 Si0 15 to 30 Ag O 1 to 20 In each of the above examplesof the glass frit, the silver oxide can be replaced by similar amountsof either a gold containing material, such as a gold resinate, or acopper containing material, such as a copper oxide.

The following examples are given to illustrate certain specificcompositions of the glass frit which can be used in the enamelresistance material of the present invention and the method of makingthe compositions, it being understood that the details of the examplesare not to be taken as in any way limiting the invention thereto.

EXAMPLE I A lead borosilicate glass frit containing silver ions isprepared from a raw batch containing Grams PbO 78 B203 6 Si0 17 Ag O Theraw batch materials are mixed in a ball mill and then placed in a fusedsilica crucible. The crucible and (n9 its contents are heated in aglobar furnace to 1200 C. and maintained at that temperature for 15 to30 minutes. The molten glass is then poured into cold water. Theresultant coarse fritted material is then ball milled to a particle sizeof preferably less than one micron.

EXAMPLE II An alkaline earth metal borosilicate glassfrit containingsilver ions is prepared from a raw batch containing Grams BaO 50 Cat) 10B 20 SiO 2O Ag O 5 The raw batch was treated in the same manner as theraw batch of Example I except that it was heated to a temperature of1260 C. to 1280 C. and maintained at that-temperature for 30 minutes.

To form the enamel resistance material of the present invention, theglass frit of the present invention, which contains the silver, gold orcopper in an ionic state, is mixed with the finely divided particles ofpalladium, platinum, rhodium or mixtures thereof in the proportions byweight of 99% to 50% glass frit and 1% to 50% conductive particles. Theamount of the conductive particles in the resistance material determinesthe conductivity of the material with the conductivity increasing withan increasing amount of the conductive particles. The conductiveparticles of the enamel resistance material of the present invention mayalso be mixed with finely divided particles of other noble metals, suchas silver and gold. For example, the enamel resistance material maycomprise the following in proportions by weight:

Percent Glass frit of the present invention 50 to 84 Palladium 11 to 28Silver 8 to 22 The following example is given to illustrate the mannerof making an enamel resistance material of the present invention, itbeing understood that the details of the example are not to be taken asin any way limiting the invention thereto.

EXAMPLEIII A resistor composition is prepared by mixing 70% by weight ofthe glass frit of the present invention of either of Examples I or II,16.8% palladium black, and 13.2% silver flake, and a vehicle of eitherbutyl carbitol acetate or 'toluol in suificient amount to obtain thedesired consistency. The mixture is ball milled for 24 to 96 hours toobtain the lowest possible average particle size, viZ., less than half amicron. The viscosity of the enamel resistancematerial is adjusted byeither removing some of the vehicle by vacuum evaporation, or by addingadditional amounts of the vehicle to permit proper application of theresistance material. The resistance material is applied to a ceramicsubstrate by either dip coating, spraying, screen printing or rollercoating, and then fired to fuse the material to the substrate. Theenamel resistance material using the glass frit of Example I is fired ata temperature of approximately 650 C., and the enamel resistancematerial using the glass frit of Example II is fired at a temperature of775 C. to 800 C.

We have found that the introduction of the silver, gold or copper in theionic state intothe glass frit improves the melting of the frit so as toimprove its wettability. In addition, it improves the overallperformance of the electrical properties of the enamel resistancematerial in that it permits the obtaining of the desired conductivityofthe material with a smaller amount of the finely divided conductor.

In an enamel type resistance material, the conductive metal particlesexist 'as isolated islands in the glassy matrix. When an electricalpotential difference is ap- 4i plied across such a resistance film,conduction takes place between the conductive metal particles throughthe glassy matrix, which glassy matrix is normally an insulator. It isfelt that the improved wettability of the glass frit of the resistancematerial of the present invention im-, proves the adherence between theglassy matrix and the conductive metal particles so as to improve theconductive properties of the resistance material.

Also, the electronic configuration of the silver, gold or copper ions inthe glass frit of the present invention is such that the ions aredeficient in an electron so that the ions can momentarily act as anelectron acceptor. However, the electronic configuration of theplatinum, palladium or rhodium metal of the conductive particles is suchthat they have a full quota of electrons and can act as an electrondonor. For this reason, it is felt that when an electrical potentialdifierence is placed across the enamel resistance material of thepresent invention, the silver, gold or copper ions in the glassy matrixact as a bridge between the isolated islands of the conductive metalparticles by accepting and passing on an electron, and thereby improvesthe conductivity of the enamel resistance material.

As an example of theimprovement in the conductivity provided by theenamel resistance material of the present invention, the enamel typeresistance materials of the compositions shown in Table I were coated ona ceramic substrate and fired at the indicated temperatures. The enamelresistance materials of Examples A and B differ in composition only bythe amount of the conductive metal particles used, with the compositionof the glass frit being the same. However, the Example C is an enamelresistance material of the present invention in that the glass frit,which is basically the same as the glass frits of Examples A and B,contains silver ions in the amount of 25% by weight of Ag 0.

As can be seen from Table I, the enamel resistance material of Example Ahas a resistance of 10 ohms per square inch, which is so high as to beconsidered nonconductive. By more than doubling the amount of theconductive metal particles in the enamel resistance material, Example B,the resistance of the material merely decreased to 3.4 1O ohms persquare inch, which is considered barely conductive. However, the enamelresistance material of'the present invention, Example C, has aresistance of 6.5 ohms per square inch, which is a high degree ofconductivity. Thus, the addition of the silver, gold or copper ions tothe glass frit not only greatly incerases the conductivity of the enamelresistance material, but does so with the use of much less of theconductive metal particles.

As previously stated, the silver, gold or copper ions are provided inthe glass frit of the present invention by the addition of the metals inthe form of oxides, resinates or the like, which materials are much lessexpensive than the substantially pure metals of the conductive metalparticles. Therefore, the reduction in the amount of the conductivemetal particles required to achieve a desired resistance value providedby the addition of the silver, gold or copper ions to the glass frit ofthe enamel resistance material of the present invention also reduces thecost of the resistance material of the present invention.

It will thus be seen that the objects set forth above,

among those made apparent from the preceding description, areefliciently attained and, since certain changes may be made in the abovecomposition of matter without departing from the scope of the invention,it is intended that all matter contained in the above description shallbe interpreted as illustrative and not in a limiting sense.

We claim:

1. A vitreous enamel resistance material adapted to be applied to andfired on a ceramic substrate to form an electrical resistor comprising afinely divided metal selected from the group consisting of palladium,platinum and rhodium, and a glass frit containing in the ionic state ametal selected from the group consisting of silver, gold and copper, allof the second said metal being in the glass in the ionic state.

2. A vitreous enamel resistance material in accordance with claim 1including 1% to 50% of the finely divided metal, and 99% to 50% of theglass frit.

3. A vitreous enamel resistance material in accordance with claim 2 inwhich the glass frit contains 1% to 25% of the metal calculated byweight of the oxide of the metal.

4. An electrical resistor comprising a ceramic substrate and a layer ofa vitreous enamel resistance material fired on the surface of saidsubstrate, said resistance material comprising a glass containing in theionic state a metal selected from the group consisting of silver, goldand copper, and finely divided metal particles selected from the groupconsisting of palladium, platinum and rhodium embedded in and dispersedthroughout the glass, all of the first said metal being in the glass inthe ionic state.

5. A resistor in accordance with claim 4 including 1% to 50% by Weightof the finely divided metal particles embedded in the glass.

6. A resistor in accordance with claim 5 in which the glass contains 1%to 25% of the metal calculated by weight of an oxide of the metal.

References Cited in the file of this patent UNITED STATES PATENTS2,515,275 Stookey July 18, 1950 2,515,936 Houston July 18, 19502,530,217 Bain Nov. 14, 1950 2,924,540 DAndrea Feb. 9, 1960 FOREIGNPATENTS 577,383 Canada June 9, 1959 855,625 Great Britain Dec. 7, 1960

1. A VITREOUS ENAMEL RESISTANCE MATERIAL ADAPTED TO BE APPLIED TO ANDFIRED ON A CERAMIC SUBSTRATE TO FORM AN ELECTRICAL RESISTOR COMPRISING AFINELY DIVIDED METAL SELECTED FROM THE GROUP CONSISTING OF PALLADIUM,PLATINUM AND RHODIUM, AND A GLASS FRIT CONTAINING IN THE IONIC STATE AMETAL SELECTED FROM THE GROUP CONSISTING OF SILVER, GOLD